This invention relates to an image-reject mixer, and in particular to an image-reject mixer which can be used in a wideband receiver.
Any receiver circuit must be designed such that it is able to reject signals in the image band. Typically, this involves using a filter, or tuned RF amplifier in the front end circuitry. In order to allow the receiver to receive signals over a wide range of frequencies, for example over the whole range from 100 MHz to 2 GHz, and to allow the image signal to be sufficiently well separated from the wanted signal, a very high first IF frequency might be chosen. However, this has the disadvantage that it is then difficult or impossible to obtain low-cost high-performance devices for use as the first local oscillator, which would have to operate at very high frequencies, or as the second local oscillator, which again would be operating at a very high frequency and requiring good stability. This arrangement also requires a high dynamic range in the front end, first mixer, first IF amplifier and second mixer.
As a result of these difficulties, it has been proposed to use image-reject mixing to suppress any signals present at the image frequency, without requiring a tuned RF amplifier.
A known image-reject mixer comprises:
first and second mixers; PA1 means for supplying first and second local oscillator signals to the first and second mixers; PA1 means for deriving first and second RF signals from an RF input signal; PA1 means for supplying the first and second RF signals to the first and second mixers, either the first and second local oscillator signals or the first and second RF signals being in quadrature with each other; and PA1 a quadrature combiner, connected to receive output signals from the first and second mixers, and to supply a mixer output signal. PA1 first and second mixers; PA1 means for supplying first and second local oscillator signals to the first and second mixers; PA1 means for deriving first and second RF signals from an RF input signal; PA1 means for supplying the first and second RF signals to the first and second mixers, either the first and second local oscillator signals or the first and second RF input signals being in quadrature with each other; and PA1 a quadrature combiner, connected to receive output signals from the first and second mixers, and to supply a mixer output signal; PA1 characterised in that the mixer further comprises: PA1 means for adding an additional input signal to the first and second RF signals; PA1 means for detecting a local oscillator signal component in the mixer output signal; and PA1 means for adjusting the signals supplied to the quadrature combiner to minimise the size of the local oscillator signal component in the mixer output signal. PA1 adjustable gain and phase elements connected to receive the output signal from one of the first and second mixers; and PA1 means for controlling the adjustable gain and phase elements on the basis of the detected local oscillator signal component in order to minimise the size of that component. PA1 a second IF mixer for generating a second IF mixer output signal from the first IF mixer output signal; PA1 a first main local oscillator for generating the first and second local oscillator signals for supply to the image-reject mixer; PA1 a second main local oscillator for generating a local oscillator signal for supply to the second IF mixer; PA1 means for mixing signals from the first and second main local oscillators to form the additional input signal at a frequency close to the image frequency.
A known image-reject mixer of this type is shown in FIG. 1. The image-reject mixer 2 includes first and second mixer circuits 4, 6. The mixers 4, 6 are driven by respective local oscillator signals which differ in phase by 90.degree.. Thus, a local oscillator signal is supplied to a splitter 8, which feeds a first local oscillator signal to the first mixer 4, and a second local oscillator signal, in quadrature with the first, to the second mixer 6. The RF input signal, including the wanted signal together with an image signal, is supplied to a second splitter 10, which feeds the first and second mixers 4, 6. The in-phase splitter 10 may easily be realised for broadband operation by either transformer or resistive splitting techniques; the former is preferable, because the latter has the disadvantage of an additional 3 dB degradation in the mixer noise figure. The output signals from the first and second mixers 4, 6 are supplied to a quadrature combiner 12, that is a device which adds one input to another, with a 90.degree. phase shift in one of the inputs. The effect of this is that the image signal is cancelled in the IF output, and appears in a second output which can be resistively terminated. (If desired, the image signal can be supplied as the output).
The quadrature combiner 12 operates at the IF frequency, and therefore only needs to be a narrow band component, which can use transformer, microstrip, Wireline (Trademark) or lumped-element techniques.
The mixers 4, 6 need to have a good dynamic range, such as can be achieved with high-level diode ring mixers, but may not be achievable with existing IC mixers.
It should be noted that the local oscillator signal could be supplied to the splitter 10, with the RF input signal being supplied in quadrature to the mixers 4, 6 through the splitter 8. However, the illustrated circuit has the advantage that it is simpler to design the required broadband 90.degree. splitter for a constant-level high strength signal such as the local oscillator signal. For example, as is well known, the splitter could be realised by the use of a higher LO frequency divided down by ECL dividers.
Using such a prior art image-reject mixer, it is possible to achieve an image rejection in the region of 20 dB. As such, prior art image-reject mixers are not suitable for use as the only image rejection mechanism in a general purpose receiver, in which image rejection of at least 60 dB may be required.